Generally, a lithographic printing plate is prepared from a light-sensitive lithographic printing plate which is called a presensitized (PS) plate.
Lithographic printing is a printing system taking advantage of a property that water and oil are essentially immiscible, and the surface of the printing plate comprises an area which receives water and repels an oily ink and an area which repels water and receives the oily ink. The former is a non-image area and the latter is an image area. Further, a boundary chemical difference between the image area and the non-image area is broadened by moistening the non-image area with a fountain solution thereby increasing the ink-repellency in the non-image area and the ink-receptivity in the image area.
The fountain solution which has been conventionally known includes an aqueous solution containing a colloidal material such as an alkali metal salt or an ammonium salt of dichromic acid, phosphoric acid or a salt thereof, for example, an ammonium salt.
Also, a water-soluble polymeric compound as a stain preventing component is usually incorporated into the fountain solution. The water-soluble polymeric compound is effective for stable printing by preventing stains of the non-image area of the printing plate or stains of water supply rolls of the printing machine. Such water-soluble polymeric compounds include natural substances such as gum arabic, cellulose and derivatives thereof (for example, carboxymethyl cellulose, carboxyethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, ethyl cellulose), starch derivatives (for example, dextrin, enzyme-decomposed dextrin, hydroxypropylated starch, carboxymethyl starch, phosphorated starch, octenylsuccinated starch), and alginic acid and derivatives thereof (for example, hydroxypropyl alginic acid, hydroxyethyl alginic acid); and synthetic hydrophilic polymers such as polyethylene glycol and copolymers thereof, polyvinyl alcohol and copolymers thereof, polyvinylpyrrolidone, polyacrylamide and copolymers thereof, polyacrylic acid and copolymers thereof, polystyrenesulfonic acid and copolymers thereof, and a vinyl acetate/maleic anhydride copolymer.
However, the fountain solution containing only these compounds has a defect in that it is difficult to uniformly dampen the non-image area of the plate, and, thus, there are problems such that prints are sometimes stained and a skilled technique is required for adjusting the amount of the fountain solution to be supplied.
Also, gum arabic which has the most excellent performance among these compounds is produced mainly in Sudan which is one of the countries of the gum arabic belt zone in the African Continent, but in recent years the availability of gum arabic becomes to be unstable due to natural conditions such as drought and the instability in the political situation. For this reason, development of hydrophilic polymers as a substitute therefor has been strongly desired.
In particular, carboxymethyl cellulose, among others, has an ability to prevent stains similar to gum arabic, but the solubility of carboxymethyl cellulose markedly decreases in an acidic pH range where the fountain solution is generally used, and, when a running processing is conducted using a pH automatic controller, a dropping nozzle of the fountain solution tends to be clogged. Generally, a commercial product of a fountain solution is concentrated for an economical reason and is diluted with water when it is used for printing. In such a case, concentration of a fountain solution containing carboxymethyl cellulose is difficult to attain because of its low solubility.
In the present invention, the concentrated fountain solution is hereinafter referred to as a composition for a fountain solution.
In addition, a Dahlgren system using an aqueous solution containing isopropyl alcohol in an amount of from about 20 to 25% as a fountain solution has been proposed. This system is advantageous in that wettability in the non-image area can be improved and adjustment in the balance of supplying amounts of printing ink and water is easy.
However, since isopropyl alcohol tends to easily evaporate, a particular apparatus is required for maintaining the concentration of isopropyl alcohol at a constant value thereby increasing the cost of the system. Further, the use of isopropyl alcohol is undesirable from the standpoint of the working environment since it has an unpleasant odor and also has a toxic problem. Furthermore, when isopropyl alcohol and gum arabic are used together, emulsification is decreased and a composition having a high concentration over 15% cannot be obtained.
Recently, social concern for industrial pollution has been increased, and exhaust of chromium ions in waste has become strictly regulated, and also the use of organic solvents such as isopropyl alcohol tends to be restricted from a safety standpoint. For this reason, a fountain solution containing no chromium ions and isopropyl alcohol has been desired.
In order to solve the above problems, compositions for a fountain solution containing various surface active agents have been proposed in, for example, JP-B-55-25075, JP-B-55- 19757 and JP-B-58-5797 (the term "JP-B" as used herein means an examined Japanese patent publication), but these compositions are still unsatisfactory for solving the above problems. Further, the fountain solution containing these surface active agents have problems in that they tend to foam when the fountain solution is transported by pumping or stirred.
On the other hand, U.S. Pat. No. 3,877,372 discloses a fountain solution containing a mixture of ethylene glycol monobutyl ether and at least one of hexylene glycol and ethylene glycol. Also, U.S. Pat. No. 4,278,467 discloses a fountain solution containing at least one of n-hexoxydiethylene glycol, n-hexoxyethylene glycol, 2-ethyl-1,3-hexanediol, n-butoxyethylene glycol acetate, n-butoxydiethylene glycol acetate, and 3-butoxy-2-propanol. JP-A-57-199693 discloses a fountain solution containing 2-ethyl-1,3-hexanediol and at least one of completely water-soluble propylene glycol, ethylene glycol, dipropylene glycol, diethylene glycol, hexylene glycol, triethylene glycol, tetraethylene glycol, tripropane glycol and 1,5-pentanediol. (The term "JP-A" as used herein means an unexamined published Japanese patent application.) These fountain solutions are advantageous in the safety aspect because of the absence of isopropyl alcohol, but have a problem in that they exhibit insufficient dampening of non-image areas during the printing in the PS plate using an anodically oxidized aluminum substrate, and, thus, in high speed printing, these fountain solutions may cause stains in the non-image areas and also cause a so-called "ink spreading" of dot image area, i.e., non-uniformity in the shape of dots in the image area.
However, these substitute solvents for isopropyl alcohol have been considered to be difficult to put into practical use since they generally have a low ability of for dissolution, and, particularly in a concentrated fountain solution, when water-soluble polymers are dissolved in cold or warm water, swollen particles of the polymers are aggregated to form a viscous mass called "undissolved powder lump" which is difficult to dissolve and which adversely affects workability. That is, these substitutes do not satisfy various characteristics required for a lithographic printing plate fountain solution substituting for gum arabic and still have serious defects.